The G28 G-code can mean different things depending on whether you are using a CNC mill, or a 3D printer.
This is because when the Reprap project chose it's Gcodes, and what they are supposed to do, they completely ignored the existing G-code standard, and messed everything up.
Not our fault !
On a Reprap, the G28 Gcode means “go to the endstops and stop moving when you have hit them”.
On a CNC mill ( traditionally ), the G28 G-code means “go to the recorded origin point for the machine”.
In the Smoothie firmware, if you set grbl_mode to true, it will have the “CNC mill” meaning, and if you set it to false ( or do not set it at all ), it will have the Reprap meaning. See grbl-mode.
To see information on the G28 Gcode for CNC mills, see G28-cnc
To see information on the G28 Gcode for Reprap machines, see G28-reprap